Answer: The required time is 12 hours.
Step-by-step explanation: Given that Shelly biked 21 miles in 4 hours.
We are to find the time taken by Shelly to bike 63 miles.
We will be using the UNITARY method to solve the problem.
We have
Time taken by Shelly to bike 21 miles = 4 hours.
So, time taken by Shelly to bike 1 mile will be

Therefore, time taken by Shelly to bike 63 miles is given by

Thus, the required time is 12 hours.